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Differential molecular beam epitaxy for multilayered bipolar devices
Thin Solid Films, 184 (1990) 253 260
253
D I F F E R E N T I A L M O L E C U L A R BEAM E P I T A X Y FOR M U L T I L A Y E R E D BIPOLAR DEVICES U. Kt~NIG, M. KUISL, J. F. LUY, H. KIBBEL AND F. SCH,~FFLER
AEG Research Center Ulm, Sedanstrafle 10, D 7900 Ulm (F.R.G) (Received May 30, 1989)
Patterned multilayers are deposited by means of differential silicon MBE. Simultaneously monocrystalline islands are formed in a polycrystalline embedment. The structure is investigated with respect to the poly/mono-boundary and the polysilicon. The poly-embedment can work as an isolation or as an interconnection for a mono-island. Several devices have been realized, namely differential diodes, a differential three-dimensional IMPATT, and a differential resonant tunnel diode and these demonstrate the feasibility of the differential multilayer silicon MBE.
1. INTRODUCTION
Differential molecular beam epitaxy (MBE) has been introduced 1'2 as a technique that allows lateral structuring of MBE layers during the growth process itself. If silicon is deposited on a silicon substrate which is covered by a prepatterned oxide layer, epitaxial growth occurs only in the oxide windows, which expose the clean substrate, while polycrystalline silicon forms on the oxide-covered areas. This technique has already been used for the realization of discrete devices 2 and even for complex-integrated bipolar circuits 3. In any case, only a single, usually low-doped layer was deposited. If several active layers are needed for devices in the monosilicon, doping was carried out conventionally by implantation. We have investigated the direct differential MBE deposition of all layers required for active devices. Since monocrystalline multilayers in the window and polycrystalline multilayers on the oxide are in contact, special attention is paid to the properties of the polysilicon and the poly/mono-boundary. 2. TECHNOLOGY AND ANALYTICS For differential MBE the silicon wafer is covered first with a thermally grown SiO2 film (70 nm-1 lam thick). Windows, usually circular 50 Ixm in diameter, are defined by lithography and opened either by plasma etching or with a buffered H F solution. An additional RCA cleaning procedure with a final H F dip is performed in order to improve the surface quality in the window. 0040-6090/90/$3.50
© ElsevierSequoia/Printedin The Netherlands
254
u. KONIGet al.
The wafer is introduced immediately into the MBE system (Atomika) 4. The MBE process starts with a thermal heat treatment (850-900 °C, 1-5 rain) to remove the native oxide in the windows. The deposition temperature for novel differential device structures is between 450 and 550 °C and for material investigations between 350 and 750°C. Different doping procedures are investigated, namely adlayercontrolled doping (AC), flux-controlled doping (FC) 5, and solid phase doping (SP) 6. The doping range reaches from 1015 c m - 3 to 6 x 1019 c m 3, n-type (antimony) or ptype (gallium). With differential MBE a monocrystalline multilayer L1 to L. grows in the window. Simultaneously a polycrystalline layer sequence L'I to L'. grows on the surrounding SiO2 (see Fig. l(a)).
;i~~ii~;;?~
Fig. l. Princip~e~fdi~erentia~MBE.Simu~tane~usdep~siti~n~fam~n~-~ayersequenceL1-"andap~y~ layer sequence L'1_.. Dependenceon the thickness of the oxide window is shown schematically(a) and in cross-sections (b). In general, layer sequences deposited for bipolar device structures have a heavily doped contact layer on top, p + for the devices reported here. In order to separate monocrystalline islands by means of the polycrystalline embedment, the well conducting polycrystalline p+-layer is usually removed wet chemically by a H N O 3/HF solution. As an alternative process for eliminating the p +-poly-influence we investigate the amorphization of the p+-poly around the monosilicon island. For this purpose silicon is implanted with a multienergy 30/60/90 keV at a dose of 1-2 × 1015 cm 2. During implantation the monosilicon is partly capped by the contact covered with a photo resist. N o subsequent annealing is performed. For an ohmic contact on top of a layer sequence titanium (50 nm), and gold (250nm) are evaporated and patterned to 3 0 g m in diameter, smaller than the diameter of the monocrystalline island. The backside of the wafer usually forms the second device contact. This arrangement is used for d.c. measurements of diodes. For r.f. measurements the substrate is wet chemically ( K O H ) thinned from the backside to about 5 gm. Ti/Au is evaporated onto the fresh backside. Only then is
DIFFERENTIAL MBE FOR MULTILAYERS
255
the top contact evaporated and patterned on the monosilicon. The thin foil is cleaved and mounted upside down into a standard microwave diode package 7. Differentially grown layers are characterized with respect to the boundary between the active monocrystalline island and the surrounding polycrystalline material. Mono/poly-boundaries are made visible by means of bevelled (min 18') cross-sections. Buffered HF and/or perjodic acid is used to reveal interfaces in multilayers. The electrical properties of polysilicon and monosilicon are evaluated using a four-point probe and a special resistor structure patterned on the layers s. The performance of differential, two-terminal devices is analysed by measuring I-Vcurves using a parameter analyser. Novel differential IMPATT diodes are tested in a waveguide in a pulsed operation mode. The current pulse length is about 20-30 ns, but was not adjusted to a rectangular shape. The duty cycle is about 1:350. The r.f. signal was detected employing a silicon-Schottky diode detector 9. 3.
PROPERTIES OF DIFFERENTIAL MOLECULAR BEAM EP1TAXIAL LAYERS
With the differential MBE process reported here all active device layers L x_. are grown together with a respective polycrystalline layer sequence L' 1 _. around. Critical layer interfaces such as p-n junctions or Si/SiGe heterojunctions pierce through the poly/mono-boundary around the active device island for thick as well as for thin oxide underneath the polysilicon. This differs from the process reported in the literature 3, where only a single low-doped layer was deposited in an oxide window and where the doping structure required for active devices has been implanted into this island. Consequently the demands on the quality of the poly/mono-boundary and on the polysilicon itself are much higher in the present case. Cross-sections in Fig. 1 through differential MBE structures give information on the poly/mono-boundary which is found to be relatively sharp at low growth temperatures. It roughly depends on the grain size of the poly-material, which is quite large (0.3 Ixm) in the pictured case of a high deposition temperature (750 °C). The boundary begins at the foot of the SiO2 layer. Important for device patterning is that the boundary extends into the mono-zone. Consequently a sub micrometer patterning ofa mono-zone with differential MBE is only possible when working at a low deposition temperature, where the grain size is small 8. In the case of a thick oxide with a steep edge (see Fig. l(b)) the poly-layers are partly interrupted at the foot of the oxide. When highly doped polysilicon is used as an interconnection to monosilicon islands this works perfectly for a smooth oxide edge. According to Fig. 1 an oxide, thinner than the thinnest layer in the sequence is most desirable in this respect. Furtheron, a quasiplanar surface can be achieved with a thin oxide, where the surface step corresponds to the oxide thickness. Concerning electrical properties of the poly/mono-boundary we refer to the interpretation of characteristics of differential devices discussed in Section 5. Thickness, composition and average doping of the monocrystalline layers L, and the respective polycrystalline layers L', are basically the same, apart from some differences in the doping profiles 8. The resistivities, however, differ distinctly. That follows from the resistivities of MBE-grown polysilicon plotted in Fig. 2. The values
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are between 1 0 - 1 Q c m for heavy doping and 2 x l 0 S ~ c m for low doping. Comparable monocrystalline MBE layers have resistivities from 5 x 10- 3 ~ cm to 1 fZ cm. Polysilicon is semi-insulating up to a doping level of 10 ~8 cm- 3 and changes above that level rapidly into a well-conducting material. The resistivities of p-type AC- and FC-doped polysilicon layers nearly coincide. SP-doped layers have higher resistivities. The resistivities are independent of the conductivity type of the
106
I,
10s 10~,
103 10z 10I
10-1
10-2 10-j
io~ lo' 1~" lo' LI~" 1'o" lo" [3opin 9 (cm "3)
Fig. 2. Doping dependence of the resistivities of polysilicon for several doping procedures AC, FC, SP (see text). TABLE I CRI T E RI A F O R D O P I N G MULT1LAYERED DEVICES P A T T E R N E D BY MEANS OF D I F F E R E N T I A L MBE. 1T D E P E N D S O N THE D O P I N G LEVEL A N D T H E D O P I N G P R O C E D U R E ( A C O R FC O R SP) OF A N A C T I V E DEVICE LAYER, W H E T H E R T H E RESPECTIVE P O L Y - L A Y E R ACTS AS A S E P A R A T I O N O R AS A N I N T E R C O N N E C T I O N .
Active mono-island Doping
Surrounding poly Device applications
Poly-functions
examples
"ii!i!::iii::i~ - ' - ~ f ~ i : ; i ' i .-.-°%-.°..11
isolation
n , p < 101Scm 3
Collector conv. Trans Emitter, Collector HBT 2DLayerHEMT Driftzone IMPATT Quantum Well RTD Absorption P I N P h D
i::::::::::.
interconnection
Ac/ FC Doping SPJ
n, p ~ 1018 _ 1019 c m - 3
Emitter conv. Trans Emitter, Base HBT Mod. doped Layer H E M T SP Doping Cladding Layer RTD
n , p > 1019cm 3
Emitter conv. Trans Base H BT Contact Layers Intermediate Layers 3D
!}:!i!!iii!!:
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AC ] Doping FC
AC/ AC n-type?
FC Doping SP J
DIFFERENTIAL MBE FOR MULTILAYERS
257
layer for FC and SP. Both different doping profiles and different grain sizes are responsible for the observed dependence of the doping procedure. 4.
DIFFERENTIAL DEVICES
The above-mentioned properties of differential MBE layers reveal that the resistivity of low-doped polysilicon layers seems sufficiently high to be used for a lateral isolation of devices, while, in contrast, highly doped polysilicon could be used as leads for the interconnections of devices (see insets in Table I). However, in a standard process, the doping in the polysilicon area is governed by the doping requirements of the monocrystalline active device. Criteria for the appropriate doping of novel differential devices are listed in Table I. Low-doped layers find application in numerous devices. The respective poly-layers, which are in contact with the mono-layers, are only in this case sufficiently insulating. The situation is indefinite, if a doping between 1018 and 1 0 1 9 c m - 3 is required, because it corresponds to the transition region from semiinsulating to conducting polysilicon. If in this case a well conducting poly-layer is required, AC or FC doping should be applied. If heavily doped layers are required in devices the surrounding poly-layers are well conducting, too, and can be used as leads. Several novel device structures have been realized with differential MBE, see cross-sections in Fig. 3. Two types of differential bipolar diodes are presented. In the case of the differential diode within thin oxide (0.1 p.m) it is clearly visible that the monocrystalline n-p-p ÷ layer sequence is in contact with the respective polycrystalline layer sequence. Owing to the low doping of the n- and p-type layers (2 × 101 v cm- 3) the surrounding poly-layers yield an isolation. The p +-poly-layer is still left around the p ÷-mono-layer. In the case of the differential diode within thick oxide (1 lam) the monosilicon seems to be spatially separated from the polysilicon. However, from different cross-sectional preparations we deduce that the mono and poly zones are connected by a thin poly-layer running over the steep oxide edge, similarly as shown in Fig. l(b). A differential three-dimensional device, a 3D-IMPATT 1o, with layer sequences of two IMPATTs I1 and 12 monolithically stacked, is shown in Fig. 3. A special twostep differential MBE process is applied 11. Again, the doping levels of the essential device layers are below the insulating--conducting transition. The absence of a visible layer sequence in the polysilicon is an indication of homogeneous polysilicon, working as an insulating surrounding for the mono-zone. A differential resonant tunnel diode (RTD) with a complex Si/SiGe layer sequence is shown in Fig. 3. The doping of the layers is unintentionally p - ~ 1015 cm-3 apart from an uppermost p+ ~ 3 × 1 0 1 9 c m - 3 layer. The p--poly-layers are semi-insulating, while the p+poly-layer has a low resistance. Nevertheless, the p+-poly-layer remains in this case around the mono-island. 5. PROPERTIES OF DIFFERENTIAL DEVICES
Several reverse I - V curves of differential diodes are shown in Fig. 4. Diodes
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"nono/po[y bounaary
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Fig. 3. Bevelled section of several multilayered devices realized by means of differential MBE.
as deposited
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Fig. 4. Methods of eliminating the influence of a heavily doped poly-layer in a diode and respective ~ V curves: with p + poly, p + poly etched, p + poly amorphized. The monosilicon has a diameter of 50 p-m, the contact a diameter of 30 ~tm.
259
DIFFERENTIAL MBE FOR MULTILAYERS
with the same diameter of the mono-zone are compared here. Methods of eliminating the influence of a heavily doped poly-layer around the mono-island are demonstrated in this figure, too. The reverse current is intolerably high with p +-poly around. The leakage current flows within the surrounding poly-material parallel to the p - n junction of the diode. After removing the p +-poly, as sketched in the figure, the reverse current is drastically reduced. As the leakage current is expected to depend on the poly/mono-boundary, this result is regarded as an indication of a sufficiently good boundary. For a detailed analysis of the leakage current along the boundary the I - V curves of diodes with varying diameters are presently measured. A new method for the separation of a mono-island is to amorphize the surrounding p +-poly. It has the advantage of a more planar surface. The measured I - V c u r v e (see Fig. 4) looks like the curve in the case of a device with a p + -poly-layer removed, with respect to reverse current (of about 10-4mA) and break-down voltage (of about 16 V). Consequently, this seems to be a promising method, which is about to be investigated in more detail. As a further feasibility test for the differential MBE technique we have investigated differential diodes with respect to their application as microwave devices. The layer sequence in the mono-zone corresponds to typical I M P A T T devices (e.g. refs. 7 and 9). For the first time oscillations at about 103 G H z (see Fig. 5) could be achieved with those novel devices. The r.f. power is found to be much lower than for conventional mesa-like I M P A T T s . This is due to much higher capacities and thermal resistances: the mounting is at the m o m e n t not adapted to the novel device structure.
,j' heat
(a)
sink
j
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/~ (b)
Fig. 5. A novel I M P A T T diode mounted in a microwave package as sketched in (a) and (b), an r.f. result, which demonstrates the feasibility of the differential MBE technique.
I - V curves of the differential 3 D - I M P A T T s and the differential R T D s - - a l s o shown in Fig. 3 - - a r e presented elsewhere 11,12. According to the d.c. results of the 3 D - I M P A T T this new device works like two single diodes in series. The results with the new type of R T D having even a p+-poly-layer around the active island are similar to a conventional mesa-like RTD. The results of both devices thus point again to a tolerable leakage currrent and thus to the quality of the poly/monoboundary.
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U. KOMG et al.
6. CONCLUSION The characteristics of the differential MBE investigated is that layer sequences for active devices can be grown in an oxide window with a respective polycrystalline layer sequence on the oxide. Owing to the fact that the mono-island and the polyembedment are grown simultaneously, the freedom in tailoring the polymaterial to a separation or to an interconnection purpose is restricted. However, it is shown that one can find compromises by choosing the appropriate doping level, the appropriate doping procedure and/or the appropriate patterning method. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
Y. Ota, Thin Solid Films, 106 (1983) 3. H.J. Herzog and E. Kasper, J. Electrochem Soc., 132 (1985) 2227. E. Kasper, H. J. Herzog and K. W6rner, J. Cryst. Growth, 81 (1987) 458. E. Kasper, H. Kibbel and F. Sch/iffler, J. Electrochem. Soc., 136 (1989) 1154. F.G. Allen, S. S. Iyer and R. A. Metzger, Applie. Surf Sci., 11/12 (1982) 517. D. Streit, R. A. Metzger and F. G. Allen, Appl. Phys. Lett., 44 (1984) 234. J.F. Luy, H. Kibbel and E. Kasper, Int. J. Inf. and Millim. Waves, 7 (1986) 305, M. Kuisl. U. K6nig, F. Sch/iffler and R. Lossos, Springer Proc. in Physics, 35 (1989) 192. J.F. Luy, 184(1990) 185. U. K6nig, M. Kuisl, J. F. Luy and H. Kibbel, Springer Proc. in Physics, 35 (1989) 376. U. K6nig, E. Kasper and J. F. Luy, GME Fachbericht 4, VDE Verlag, Berlin, 1989, p. 121. U. K6nig, M. Kuisl, J. F. Luy and F. Sch/iffler, Electronics Letters, 25 (1989) 1169.
253
D I F F E R E N T I A L M O L E C U L A R BEAM E P I T A X Y FOR M U L T I L A Y E R E D BIPOLAR DEVICES U. Kt~NIG, M. KUISL, J. F. LUY, H. KIBBEL AND F. SCH,~FFLER
AEG Research Center Ulm, Sedanstrafle 10, D 7900 Ulm (F.R.G) (Received May 30, 1989)
Patterned multilayers are deposited by means of differential silicon MBE. Simultaneously monocrystalline islands are formed in a polycrystalline embedment. The structure is investigated with respect to the poly/mono-boundary and the polysilicon. The poly-embedment can work as an isolation or as an interconnection for a mono-island. Several devices have been realized, namely differential diodes, a differential three-dimensional IMPATT, and a differential resonant tunnel diode and these demonstrate the feasibility of the differential multilayer silicon MBE.
1. INTRODUCTION
Differential molecular beam epitaxy (MBE) has been introduced 1'2 as a technique that allows lateral structuring of MBE layers during the growth process itself. If silicon is deposited on a silicon substrate which is covered by a prepatterned oxide layer, epitaxial growth occurs only in the oxide windows, which expose the clean substrate, while polycrystalline silicon forms on the oxide-covered areas. This technique has already been used for the realization of discrete devices 2 and even for complex-integrated bipolar circuits 3. In any case, only a single, usually low-doped layer was deposited. If several active layers are needed for devices in the monosilicon, doping was carried out conventionally by implantation. We have investigated the direct differential MBE deposition of all layers required for active devices. Since monocrystalline multilayers in the window and polycrystalline multilayers on the oxide are in contact, special attention is paid to the properties of the polysilicon and the poly/mono-boundary. 2. TECHNOLOGY AND ANALYTICS For differential MBE the silicon wafer is covered first with a thermally grown SiO2 film (70 nm-1 lam thick). Windows, usually circular 50 Ixm in diameter, are defined by lithography and opened either by plasma etching or with a buffered H F solution. An additional RCA cleaning procedure with a final H F dip is performed in order to improve the surface quality in the window. 0040-6090/90/$3.50
© ElsevierSequoia/Printedin The Netherlands
254
u. KONIGet al.
The wafer is introduced immediately into the MBE system (Atomika) 4. The MBE process starts with a thermal heat treatment (850-900 °C, 1-5 rain) to remove the native oxide in the windows. The deposition temperature for novel differential device structures is between 450 and 550 °C and for material investigations between 350 and 750°C. Different doping procedures are investigated, namely adlayercontrolled doping (AC), flux-controlled doping (FC) 5, and solid phase doping (SP) 6. The doping range reaches from 1015 c m - 3 to 6 x 1019 c m 3, n-type (antimony) or ptype (gallium). With differential MBE a monocrystalline multilayer L1 to L. grows in the window. Simultaneously a polycrystalline layer sequence L'I to L'. grows on the surrounding SiO2 (see Fig. l(a)).
;i~~ii~;;?~
Fig. l. Princip~e~fdi~erentia~MBE.Simu~tane~usdep~siti~n~fam~n~-~ayersequenceL1-"andap~y~ layer sequence L'1_.. Dependenceon the thickness of the oxide window is shown schematically(a) and in cross-sections (b). In general, layer sequences deposited for bipolar device structures have a heavily doped contact layer on top, p + for the devices reported here. In order to separate monocrystalline islands by means of the polycrystalline embedment, the well conducting polycrystalline p+-layer is usually removed wet chemically by a H N O 3/HF solution. As an alternative process for eliminating the p +-poly-influence we investigate the amorphization of the p+-poly around the monosilicon island. For this purpose silicon is implanted with a multienergy 30/60/90 keV at a dose of 1-2 × 1015 cm 2. During implantation the monosilicon is partly capped by the contact covered with a photo resist. N o subsequent annealing is performed. For an ohmic contact on top of a layer sequence titanium (50 nm), and gold (250nm) are evaporated and patterned to 3 0 g m in diameter, smaller than the diameter of the monocrystalline island. The backside of the wafer usually forms the second device contact. This arrangement is used for d.c. measurements of diodes. For r.f. measurements the substrate is wet chemically ( K O H ) thinned from the backside to about 5 gm. Ti/Au is evaporated onto the fresh backside. Only then is
DIFFERENTIAL MBE FOR MULTILAYERS
255
the top contact evaporated and patterned on the monosilicon. The thin foil is cleaved and mounted upside down into a standard microwave diode package 7. Differentially grown layers are characterized with respect to the boundary between the active monocrystalline island and the surrounding polycrystalline material. Mono/poly-boundaries are made visible by means of bevelled (min 18') cross-sections. Buffered HF and/or perjodic acid is used to reveal interfaces in multilayers. The electrical properties of polysilicon and monosilicon are evaluated using a four-point probe and a special resistor structure patterned on the layers s. The performance of differential, two-terminal devices is analysed by measuring I-Vcurves using a parameter analyser. Novel differential IMPATT diodes are tested in a waveguide in a pulsed operation mode. The current pulse length is about 20-30 ns, but was not adjusted to a rectangular shape. The duty cycle is about 1:350. The r.f. signal was detected employing a silicon-Schottky diode detector 9. 3.
PROPERTIES OF DIFFERENTIAL MOLECULAR BEAM EP1TAXIAL LAYERS
With the differential MBE process reported here all active device layers L x_. are grown together with a respective polycrystalline layer sequence L' 1 _. around. Critical layer interfaces such as p-n junctions or Si/SiGe heterojunctions pierce through the poly/mono-boundary around the active device island for thick as well as for thin oxide underneath the polysilicon. This differs from the process reported in the literature 3, where only a single low-doped layer was deposited in an oxide window and where the doping structure required for active devices has been implanted into this island. Consequently the demands on the quality of the poly/mono-boundary and on the polysilicon itself are much higher in the present case. Cross-sections in Fig. 1 through differential MBE structures give information on the poly/mono-boundary which is found to be relatively sharp at low growth temperatures. It roughly depends on the grain size of the poly-material, which is quite large (0.3 Ixm) in the pictured case of a high deposition temperature (750 °C). The boundary begins at the foot of the SiO2 layer. Important for device patterning is that the boundary extends into the mono-zone. Consequently a sub micrometer patterning ofa mono-zone with differential MBE is only possible when working at a low deposition temperature, where the grain size is small 8. In the case of a thick oxide with a steep edge (see Fig. l(b)) the poly-layers are partly interrupted at the foot of the oxide. When highly doped polysilicon is used as an interconnection to monosilicon islands this works perfectly for a smooth oxide edge. According to Fig. 1 an oxide, thinner than the thinnest layer in the sequence is most desirable in this respect. Furtheron, a quasiplanar surface can be achieved with a thin oxide, where the surface step corresponds to the oxide thickness. Concerning electrical properties of the poly/mono-boundary we refer to the interpretation of characteristics of differential devices discussed in Section 5. Thickness, composition and average doping of the monocrystalline layers L, and the respective polycrystalline layers L', are basically the same, apart from some differences in the doping profiles 8. The resistivities, however, differ distinctly. That follows from the resistivities of MBE-grown polysilicon plotted in Fig. 2. The values
256
u. K6NZG et al.
are between 1 0 - 1 Q c m for heavy doping and 2 x l 0 S ~ c m for low doping. Comparable monocrystalline MBE layers have resistivities from 5 x 10- 3 ~ cm to 1 fZ cm. Polysilicon is semi-insulating up to a doping level of 10 ~8 cm- 3 and changes above that level rapidly into a well-conducting material. The resistivities of p-type AC- and FC-doped polysilicon layers nearly coincide. SP-doped layers have higher resistivities. The resistivities are independent of the conductivity type of the
106
I,
10s 10~,
103 10z 10I
10-1
10-2 10-j
io~ lo' 1~" lo' LI~" 1'o" lo" [3opin 9 (cm "3)
Fig. 2. Doping dependence of the resistivities of polysilicon for several doping procedures AC, FC, SP (see text). TABLE I CRI T E RI A F O R D O P I N G MULT1LAYERED DEVICES P A T T E R N E D BY MEANS OF D I F F E R E N T I A L MBE. 1T D E P E N D S O N THE D O P I N G LEVEL A N D T H E D O P I N G P R O C E D U R E ( A C O R FC O R SP) OF A N A C T I V E DEVICE LAYER, W H E T H E R T H E RESPECTIVE P O L Y - L A Y E R ACTS AS A S E P A R A T I O N O R AS A N I N T E R C O N N E C T I O N .
Active mono-island Doping
Surrounding poly Device applications
Poly-functions
examples
"ii!i!::iii::i~ - ' - ~ f ~ i : ; i ' i .-.-°%-.°..11
isolation
n , p < 101Scm 3
Collector conv. Trans Emitter, Collector HBT 2DLayerHEMT Driftzone IMPATT Quantum Well RTD Absorption P I N P h D
i::::::::::.
interconnection
Ac/ FC Doping SPJ
n, p ~ 1018 _ 1019 c m - 3
Emitter conv. Trans Emitter, Base HBT Mod. doped Layer H E M T SP Doping Cladding Layer RTD
n , p > 1019cm 3
Emitter conv. Trans Base H BT Contact Layers Intermediate Layers 3D
!}:!i!!iii!!:
T | ~ . |~; •
AC ] Doping FC
AC/ AC n-type?
FC Doping SP J
DIFFERENTIAL MBE FOR MULTILAYERS
257
layer for FC and SP. Both different doping profiles and different grain sizes are responsible for the observed dependence of the doping procedure. 4.
DIFFERENTIAL DEVICES
The above-mentioned properties of differential MBE layers reveal that the resistivity of low-doped polysilicon layers seems sufficiently high to be used for a lateral isolation of devices, while, in contrast, highly doped polysilicon could be used as leads for the interconnections of devices (see insets in Table I). However, in a standard process, the doping in the polysilicon area is governed by the doping requirements of the monocrystalline active device. Criteria for the appropriate doping of novel differential devices are listed in Table I. Low-doped layers find application in numerous devices. The respective poly-layers, which are in contact with the mono-layers, are only in this case sufficiently insulating. The situation is indefinite, if a doping between 1018 and 1 0 1 9 c m - 3 is required, because it corresponds to the transition region from semiinsulating to conducting polysilicon. If in this case a well conducting poly-layer is required, AC or FC doping should be applied. If heavily doped layers are required in devices the surrounding poly-layers are well conducting, too, and can be used as leads. Several novel device structures have been realized with differential MBE, see cross-sections in Fig. 3. Two types of differential bipolar diodes are presented. In the case of the differential diode within thin oxide (0.1 p.m) it is clearly visible that the monocrystalline n-p-p ÷ layer sequence is in contact with the respective polycrystalline layer sequence. Owing to the low doping of the n- and p-type layers (2 × 101 v cm- 3) the surrounding poly-layers yield an isolation. The p +-poly-layer is still left around the p ÷-mono-layer. In the case of the differential diode within thick oxide (1 lam) the monosilicon seems to be spatially separated from the polysilicon. However, from different cross-sectional preparations we deduce that the mono and poly zones are connected by a thin poly-layer running over the steep oxide edge, similarly as shown in Fig. l(b). A differential three-dimensional device, a 3D-IMPATT 1o, with layer sequences of two IMPATTs I1 and 12 monolithically stacked, is shown in Fig. 3. A special twostep differential MBE process is applied 11. Again, the doping levels of the essential device layers are below the insulating--conducting transition. The absence of a visible layer sequence in the polysilicon is an indication of homogeneous polysilicon, working as an insulating surrounding for the mono-zone. A differential resonant tunnel diode (RTD) with a complex Si/SiGe layer sequence is shown in Fig. 3. The doping of the layers is unintentionally p - ~ 1015 cm-3 apart from an uppermost p+ ~ 3 × 1 0 1 9 c m - 3 layer. The p--poly-layers are semi-insulating, while the p+poly-layer has a low resistance. Nevertheless, the p+-poly-layer remains in this case around the mono-island. 5. PROPERTIES OF DIFFERENTIAL DEVICES
Several reverse I - V curves of differential diodes are shown in Fig. 4. Diodes
258
u. K6NIG et al.
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259
DIFFERENTIAL MBE FOR MULTILAYERS
with the same diameter of the mono-zone are compared here. Methods of eliminating the influence of a heavily doped poly-layer around the mono-island are demonstrated in this figure, too. The reverse current is intolerably high with p +-poly around. The leakage current flows within the surrounding poly-material parallel to the p - n junction of the diode. After removing the p +-poly, as sketched in the figure, the reverse current is drastically reduced. As the leakage current is expected to depend on the poly/mono-boundary, this result is regarded as an indication of a sufficiently good boundary. For a detailed analysis of the leakage current along the boundary the I - V curves of diodes with varying diameters are presently measured. A new method for the separation of a mono-island is to amorphize the surrounding p +-poly. It has the advantage of a more planar surface. The measured I - V c u r v e (see Fig. 4) looks like the curve in the case of a device with a p + -poly-layer removed, with respect to reverse current (of about 10-4mA) and break-down voltage (of about 16 V). Consequently, this seems to be a promising method, which is about to be investigated in more detail. As a further feasibility test for the differential MBE technique we have investigated differential diodes with respect to their application as microwave devices. The layer sequence in the mono-zone corresponds to typical I M P A T T devices (e.g. refs. 7 and 9). For the first time oscillations at about 103 G H z (see Fig. 5) could be achieved with those novel devices. The r.f. power is found to be much lower than for conventional mesa-like I M P A T T s . This is due to much higher capacities and thermal resistances: the mounting is at the m o m e n t not adapted to the novel device structure.
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I - V curves of the differential 3 D - I M P A T T s and the differential R T D s - - a l s o shown in Fig. 3 - - a r e presented elsewhere 11,12. According to the d.c. results of the 3 D - I M P A T T this new device works like two single diodes in series. The results with the new type of R T D having even a p+-poly-layer around the active island are similar to a conventional mesa-like RTD. The results of both devices thus point again to a tolerable leakage currrent and thus to the quality of the poly/monoboundary.
260
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6. CONCLUSION The characteristics of the differential MBE investigated is that layer sequences for active devices can be grown in an oxide window with a respective polycrystalline layer sequence on the oxide. Owing to the fact that the mono-island and the polyembedment are grown simultaneously, the freedom in tailoring the polymaterial to a separation or to an interconnection purpose is restricted. However, it is shown that one can find compromises by choosing the appropriate doping level, the appropriate doping procedure and/or the appropriate patterning method. REFERENCES 1 2 3 4 5 6 7 8 9 10 11 12
Y. Ota, Thin Solid Films, 106 (1983) 3. H.J. Herzog and E. Kasper, J. Electrochem Soc., 132 (1985) 2227. E. Kasper, H. J. Herzog and K. W6rner, J. Cryst. Growth, 81 (1987) 458. E. Kasper, H. Kibbel and F. Sch/iffler, J. Electrochem. Soc., 136 (1989) 1154. F.G. Allen, S. S. Iyer and R. A. Metzger, Applie. Surf Sci., 11/12 (1982) 517. D. Streit, R. A. Metzger and F. G. Allen, Appl. Phys. Lett., 44 (1984) 234. J.F. Luy, H. Kibbel and E. Kasper, Int. J. Inf. and Millim. Waves, 7 (1986) 305, M. Kuisl. U. K6nig, F. Sch/iffler and R. Lossos, Springer Proc. in Physics, 35 (1989) 192. J.F. Luy, 184(1990) 185. U. K6nig, M. Kuisl, J. F. Luy and H. Kibbel, Springer Proc. in Physics, 35 (1989) 376. U. K6nig, E. Kasper and J. F. Luy, GME Fachbericht 4, VDE Verlag, Berlin, 1989, p. 121. U. K6nig, M. Kuisl, J. F. Luy and F. Sch/iffler, Electronics Letters, 25 (1989) 1169.